In a liquid chromatograph mass spectrometer (LC/MS) in which a mass spectrometer (MS) is used as a detector for a liquid chromatograph (LC), an atmospheric pressure ion source is used to ionize components in a liquid sample eluted from a column, using an electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), atmospheric pressure photoionization (APPI) or similar ionization method.
In the ESI, a high voltage of a few to several kV is previously applied to the tip of a thin nozzle through which a liquid sample is to be introduced. The high voltage creates an electric field, which causes charge separation in the liquid sample. The charge-separated liquid sample is broken into a nebulized form due primarily to attractive or repulsive coulomb forces. The resultant droplets collide with the ambient air, to be divided into finer particles. Concurrently, the solvent or mobile phase in the droplets vaporizes. During this process, the sample components (molecules or atoms of the sample) in the droplets are released from the droplets together with the electric charges and turn into gaseous ions. In the APCI, a needle electrode is placed in front of the tip of a thin nozzle through which a liquid sample is introduced. The sample components released from the droplets of the liquid sample nebulized by the heated nozzle are made to chemically react with carrier-gas ions (buffer ions) generated by corona discharge from the needle electrode, whereby the sample components are ionized. In the APPI, the sample components released from the droplets of the liquid sample nebulized by the heated nozzle are irradiated with light and thereby ionized.
In any of those ionization methods, an ion-drawing port is placed in front of the spray flow (normally, a stream of ions mixed with micro droplets of unvaporized solvent or the like) ejected from the nozzle. The ions drawn into the ion-drawing port pass through a desolvation pipe, to be transported to subsequent stages under vacuum atmosphere (see Non-Patent Literatures 1-3). The desolvation pipe, which is a heated pipe, does not only serve as a passage for transporting the ions but also has the effect of promoting vaporization of the solvent from the droplets and thereby helping the generation of gaseous ions.
To improve the ion generation efficiency in the previously described atmospheric pressure ion sources, it is necessary to quickly vaporize the solvent and mobile phase in the droplets sprayed from the nozzle. For this purpose, conventional atmospheric pressure ionization mass spectrometers have a system for supplying hot drying gas from the circumference of the ion-drawing port so as to make the spray flow come in contact with the drying gas. For example, in an atmospheric pressure ionization mass spectrometer disclosed in Patent Literatures 1 or 2, a drying-gas pipe is provided coaxially with and around the desolvation pipe so as to supply a drying gas in a ring-like shape from the supplying port at the end of the drying-gas pipe in the direction opposite to the ion-drawing direction. In another commonly known system, a plurality of drying-gas supplying ports are provided around the ion-drawing port and the drying gas is supplied from each of the drying-gas supplying ports.
In an atmospheric pressure ionization mass spectrometer described in Patent Literature 3, a drying-gas supplying port is provided in front of the ion-drawing port so that the drying gas can efficiently come in contact with the spray flow from the nozzle. Furthermore, this system disclosed in Patent Literature 3 has a means for regulating the supplying rate of the drying gas so that the flow rate of the drying gas can be regulated to maximize the ion detection efficiency.